Comment on studying the corrections to factorization in B → D ( ∗ ) X

We propose studying the mechanism of factorization in exclusive decays of the form B → D(∗)X by examining the differential decay rate as a function of the invariant mass of the light hadronic state X. If factorization works primarily due to the large Nc limit then its accuracy is not expected to decrease as the X invariant mass increases. However, if factorization is mostly a consequence of perturbative QCD then the corrections should grow with the X invariant mass. Combining data for hadronic tau decays and semileptonic B decays allows tests of factorization to be made for a variety of final states. We discuss the examples of B → D∗π+π−π−π0 and B → Dωπ. The mode B → Dωπ will allow a precision study of the dependence of the corrections to factorization on the invariant mass of the light hadronic state.